Issue 31442: assertion failures on Windows in Python/traceback.c in case of a bad io.open (original) (raw)

the following code causes an assertion failure on my Windows: import io def _bad_open(*args): return 42

io.open = _bad_open

1/0

this is because _Py_DisplaySourceLine() (in Python/traceback.c) assumes that the return value of io.open() is valid.

IIUC, this is actually a debug assertion failure in Windows code, in _get_osfhandle() (which is called by _Py_dup() (in Python/fileutils.c)). (also, on my Ubuntu VM, there is no assertion failure.)

the following code causes a similar assertion failure: import io def _bad_open1(*args): io.open = _bad_open2 raise Exception

def _bad_open2(*args): return 42

io.open = _bad_open1

1/0

this is because _Py_FindSourceFile() assumes that the return value of io.open() is valid, and returns it to _Py_DisplaySourceLine(), which also assume it is valid.

I thought about adding a check in _Py_DisplaySourceLine(), before calling PyObject_AsFileDescriptor(), such as: PyObject_IsInstance(binary, (PyObject*)&PyIOBase_Type); but I am not sure whether we should use PyIOBase_Type outside of the io module.

note that even with such a check, one could still write a _bad_open() that returns a subclass of IOBase, whose fileno() method returns a bad file descriptor. #15263 (and specifically https://bugs.python.org/issue15263#msg164731) mentions this.